Volume 13 - Corrosion Part 4 pptx

cessation of the crack growth. It is assumed that if it were not for the intervention of the corrosion product wedging the curve would proceed to an arrest. Fig. 47 Schematic of the variable effects of corrosion product wedging on SCC growth curves in a K-dec reasing test. Solid lines: measured curve. Dashed lines: estimated true curve excluding the effect of corrosion product wedging. Asterisks indicate temporary crack arrests. The threshold stress intensities determined by this method can be useful for ranking materials, but usually cannot be considered valid. Therefore, they cannot be used in design calculations based on fracture mechanics. Displays of complete V-K curves provide convenient comparisons of various materials, as shown in Fig. 48. Problems with the control of the testing procedure and of correlations with service conditions have impeded the standardization of this test method (Ref 24, 34). Fig. 48 SCC propagation rates for various aluminum alloy 7050 products. Double-beam specimens (S- L; see Fig. 28) bolt-loaded to pop-in and wetted three times daily with 3.5% NaCl. Plateau velocity a veraged over 15 days. The right-hand end of the band for each product indicates the pop-in starting stress intensity (K Io ) for the tests of that material. Data for alloys 7075-T651 and 7079-T651 are from Ref 35. Source: Ref 82 Dead-weight loading, or a simulated dead-weight loading system used in conjunction with automatic data logging equipment (Fig. 30(b), has proved to be a rigorous method for evaluating threshold stress intensities by SCC initiation (Ref 33, 34). Because crack growth results in increasing stress intensity and an increasing crack opening, corrosion product wedging is minimal, and each test usually has a definite end point (fracture). In these tests, fatigue precracked compact or modified compact specimens (Fig. 25(b)) are loaded to various initial stress intensities K Io and exposed until fracture or until completion of a designated time period (Fig. 29). The designated cut-off period should be long enough for extended initiation times and yet not long enough to allow corrosion product wedging to exert a dominant influence. The test results shown in Fig. 49 indicate that near-threshold values were reached within 1200 h, as judged by the flattening tendency of the curves. The slight downward slope of some of the curves after 1200 h may be the result of wedging by corrosion products, but this was not determined. The effect of such wedging would be to give lower estimates of the threshold stress intensity. Fig. 49 Initial stress intensity versus time to fracture for S-L (see Fig. 28 ) compact specimens of various aluminum alloys exposed to an aqueous solution containing 0.06 M sodium chloride, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4. Asteri sk indicates metallographic examination showed that SCC had started. Source: Ref 33 The testing of longitudinal (L-T, L-S in Fig. 28) and long-transverse (T-L, T-S in Fig. 28) specimens presents special problems with materials having typical directional grain structures. Stress-corrosion cracking growth is small and tends to be in the L-T plane, which is perpendicular to the plane of the precrack (Ref 36, 83). Such out-of-plane crack growth invalidates calculations of the plane-strain threshold stress intensity K ISCC . On the other hand, the testing of materials having an equiaxed grain structure also presents problems with stress intensity calculations because of gross crack branching; this would be applicable to specimens of any orientation. The most widely used corrodent for testing precracked specimens is 3.5% sodium chloride solution applied dropwise to the precrack two or (usually) three times daily (Ref 34, 35, 36, 37). This intermittent wetting technique accelerates SCC growth but it also causes troublesome corrosion of the mechanical precrack. Less corrosive corrodents that have been used include substitute ocean water (ASTM D 1141) and an inhibited salt solution containing 0.06 M sodium chloride, 0.02 M sodium dichromate, 0.07 M sodium acetate, and acetic acid to pH 4 (Ref 36, 37, 81). Some investigators have tested 7000-series alloys in distilled water (Ref 78) and in water vapor at 40 °C (104 °F) (Ref 84). Typical test durations that have been used range from 200 to 2500 h. With low-resistance alloys, both of the first two corrodents listed in the preceding paragraph ranked alloys similarly and in agreement with exposure to a seacoast and an inland industrial atmosphere. Plateau velocities in the laboratory tests were about five to ten times faster than in the seacoast atmosphere and ten times faster than in the industrial atmosphere. In these K-decreasing laboratory tests, corrosion product wedging effects dominated after exposure periods of about 200 to 800 h. The length of exposure time before the intervention of corrosion product wedging varies with several factors, including the magnitude of K Io and the inherent resistance to crevice corrosion of the test material in the corrosive environment (Ref 36, 41). Slow Strain Rate Testing Slow strain rate testing is not governed by any standards. Various aqueous solutions have been used in addition to 3.5% sodium chloride. Because the 3.5% sodium chloride solution did not appear aggressive enough for slow strain rate testing, more corrosive test mediums have been used, including oxidant additions to the sodium chloride solution or more acidic solutions, such as aluminum chloride (Ref 52, 85). In a round-robin testing program using several aluminum alloy types and several corrodents, a solution containing 3% sodium chloride plus 0.3% hydrogen peroxide was considered the most promising candidate for possible standardization (Fig. 33). Additional study is needed to determine the optimum composition of these constituents. Another promising candidate was a solution of 2% sodium chloride plus 0.5% sodium chromate having a pH of 3. Testing of Copper Alloys (Smooth Specimens) Testing in Mattsson's Solution. According to ASTM G 37 (Ref 8), a stressed test specimen must be completely and continuously immersed in an aqueous solution containing 0.05 g-atom/L of Cu 2+ and 1 g-mol/L of ammonium ion ( ) with a pH of 7.2. The copper is added as hydrated copper sulfate, and the is added as a mixture of ammonium hydroxide and ammonium sulfate. The ratio of the latter two compounds is adjusted to achieve the desired pH. Mattsson's pH 7.2 solution is recommended only for brasses (copper-zinc base alloys). This test environment may give erroneous results for other copper alloys and is not recommended. This is particularly true for alloys containing aluminum or nickel. This test environment is believed to provide an accelerated ranking of the relative or absolute degree of susceptibility to SCC for different brasses. The test environment correlates well with the corresponding service ranking in environments that cause SCC, which may be due to the combined presence of traces of moisture and ammonia vapor. The extent to which the accelerated ranking correlates with the ranking obtained after long-term exposure to environments containing corrodents other than ammonia is not known. Such environments may be severe marine atmospheres (chloride), severe industrial atmospheres (predominantly sulfur dioxide), or superheated ammonia-free steam. It is currently not possible to specify a time to failure in Mattsson's pH 7.2 solution that corresponds to a distinction between acceptable and unacceptable SCC behavior in brass alloys. Such correlations must be determined on an individual basis. Mattsson's pH 7.2 solution may also cause some stress-independent general and intergranular corrosion of brasses. Therefore, SCC failure may possibly be confused with mechanical failure induced by corrosion-reduced net cross section. This is most likely with small cross-sectional specimens, high applied stress levels, long exposure times, and SCC- resistant alloys. Careful metallographic examination is recommended for accurate determination of the cause of failure. Alternatively, unstressed control specimens can be exposed to corrosive environments in order to determine the extent to which stress-independent corrosion degrades mechanical properties. Other Testing Media. The most widely used SCC agent for copper and copper alloys is ammonia (NH 3 ) (Ref 86). The ion does not appear to cause cracking in a stable salt, such as ammonium sulfate. Cracking will occur in a salt that dissociates (such as ammonium carbonate) to form ammonia. The ion (x is usually 4 to 5) is thought to be necessary to induce SCC in copper metals (Ref 87). Amine groups also cause cracking, or are easily converted to ammonia. Amines and sulfamic acid also cause cracking. Dry ammonia does not cause SCC of brass, as demonstrated by the successful use of brass valves and gages on tanks of anhydrous ammonia. Stress-corrosion cracking of copper metals in ammonia will not occur in the absence of oxygen or an oxidizing agent. Carbon dioxide is also a requisite (Ref 88). Therefore, air rather than pure oxygen is necessary, and as a practical matter, moisture is essential. When other factors are favorable, a very small amount of NH 3 is sufficient to cause cracking. The controlling factor may therefore be moisture, because cracking may appear to be caused by the presence of a condensed moisture film. Other than ammonia, the most effective agents for causing cracking are the fumes from nitric acid or moist nitrogen dioxide. Sulfur dioxide will also crack brass; but both maximum and minimum concentration limits exist, and the reaction is slow (Ref 86). Alloy development studies have been conducted with a moist ammoniacal test atmosphere containing 80% air, 16% NH 3 , and 4% water vapor at 35 °C (95 °F). However, none of these corrodents has received the attention that ammonia has garnered (Ref 87). Historically, immersion of a copper alloy product in a mercurous nitrate solution has been used to test for residual stresses (Ref 89, 90). Because these residual stresses are possible sources of failure by SCC in other environments, some have regarded this test as a stress-corrosion test. However, it is only an indirect method of identifying SCC tendencies and does not correlate to the presence of SCC as well as test methods based on specific attack by ammonia (Ref 86). It does indicate, however, that mercury and other low-melting liquid metals can cause embrittlement and failure due to cracking. Testing of Carbon and Low-Alloy Steels Generally, steels with lower strengths are susceptible to SCC only upon exposure to a small number of specific environments, such as the hot caustic solutions encountered in steam boilers, hot nitrate solutions, anhydrous ammonia, and hot carbonate-bicarbonate solutions (Ref 91, 92). Boiler Water Embrittlement Detector Testing. Caustic cracking failures frequently originate in welded structures in the vicinity of faying surfaces, where small leaks cause soluble salts to accumulate in high local concentrations of caustic soda and silica. As a general rule, crevices or splash areas on hot metal surfaces where the concentration of dissolved soluble salts can occur are likely sites for SCC. This type of intergranular cracking failure has been produced with concentrations of sodium hydroxide as low as 5%, but a concentration of 15 to 30% is usually required at 200 to 250 °C (390 to 480 °F) to produce this phenomenon. The apparatus and procedures used to determine the embrittling or nonembrittling characteristics of the water in an operating boiler are detailed in ASTM D 807 (Ref 8). Other Testing Media. Caustic cracking occurs in digester vessels used in the chemical-processing industries, and laboratory studies have been conducted using sodium hydroxide concentrations of about 30 to 35% (Ref 93). Tests in boiling nitrate solutions have frequently been used to study the effects of composition and metallurgical variables (Ref 92). In studies of low-carbon steel in boiling nitrate solutions having different cations, solutions containing the more acidic cations in greater concentrations were found to be the most potent. This tendency is illustrated by the apparent threshold stresses for failure of a 0.05% C steel in nitrate solutions with a range of concentrations, as shown in Table 5. Table 5 Apparent threshold stress values for 0.05% C steel in nitrate solutions of varying concentrations Apparent threshold stress values at a solution concentration of: 8 N 4 N 2.5 N 1 N Nitrate solution MPa ksi MPa ksi MPa ksi MPa ksi Ammonium nitrate 14 2 21 3 48 7 83 12 Calcium nitrate 34 5 48 7 83 12 159 23 Lithium nitrate 34 5 55 8 131 19 at 2 N 159 23 Potassium nitrate 41 6 62 9 97 14 165 24 Sodium nitrate 55 8 131 19 152 22 179 26 Source: Ref 92 Cracking can be accelerated by the addition of small amounts of acid or oxidizing agents, such as potassium permanganate, manganese sulfate, sodium nitrite, and potassium dichromate, but hydroxides and other salts, particularly those forming insoluble iron products, such as sodium carbonate or sodium hydrogen phosphate, retard or prevent failure. Sodium nitrite is also a known inhibitor if the nitrite concentration is equal to that of the nitrate ion. A standard test environment has not been established, and conditions should be tailored to individual testing requirements. The ranking of a given series of alloys may vary with exposure conditions (Ref 58). Consequently, selection of a particular alloy for use in an environment that varies from that used in laboratory ranking tests may result in unexpected service failure. This tendency is illustrated by the effects of alloying additions in ferritic steels on cracking in two different environments (Fig. 50). Figure 50(a) illustrates that each of the alloying additions is beneficial in the carbonate- bicarbonate solutions, with molybdenum having the greatest effect. However, the molybdenum addition has an adverse effect in the 35% sodium hydroxide solution, although the beneficial effects of nickel and chromium additions remain the same (Fig. 50b). Although nickel additions are beneficial in the above example, a similar addition of nickel to a carbon- manganese steel produced susceptibility to SCC in boiling magnesium chloride; this did not occur in the steel without the addition of nickel (Ref 95). Fig. 50 Effect of various alloying elements on the SCC behavior of a low- alloy ferritic steel in two different corrosive environments. Behavior indicated by time to failure ratios i n a slow strain rate test. (a) Immersed in 1 N sodium carbonate plus 1 N sodium bicarbonate at 75 °C (165 °F). (b) Immersed in boiling 35% sodium hydroxide. Source: Ref 94 The use of laboratory testing media that duplicate service conditions is equally important when accelerated tests are used for quality control through the acceptance or rejection of production lots of a particular alloys. Reference 96 discusses tests of prestressing steels intended for use as concrete reinforcing bars (rebars) in which an ammonium thiocyanate solution was used to discriminate between heats of steel. Use of the carbonate-bicarbonate solutions for testing pipeline steels by the slow strain rate method revealed that the susceptibility to SCC was dependent on the electrochemical potential of the specimen surface in the test environment, as shown in Fig. 50(a). A critical range in which SCC occurred was established. The critical range varies with the test environment and alloy composition. Several tests at various carbonate-bicarbonate concentrations, temperatures, pH levels, and corrosion potentials indicated that test conditions using an impressed potential of -650 mV versus the saturated calomel electrode (SCE) and a temperature of 75 °C (165 °F) were optimal (Fig. 37). Testing of High-Strength Steels (Ref 4, 97) For steels with yield strengths greater than about 690 MPa (100 ksi) such as low-alloy and alloy steels, hot-work die steels, maraging steels, and martensitic and precipitation-hardenable stainless steels the environments that cause SCC are not specific. In many alloy systems, the phenomena of SCC and hydrogen embrittlement cracking are indistinguishable (Fig. 1). This is particularly the case in environments that contain sulfides or other promoters of hydrogen entry. Environments of major concern are natural waters for example, rainwater, seawater, and atmosphere moisture. Any of these environments may become contaminated, which significantly increases the likelihood of SCC. Contamination with hydrogen sulfide is particularly serious; consequently, the presence of hydrogen sulfide in high concentrations in salt water associated with certain deep oil wells (termed sour wells; see the article "Corrosion in Petroleum Production Operations" in this Volume) places an upper limit of approximately 620 MPa (90 ksi) on the yield strength that can be tolerated in stressed steel in such environments without cracking. Sulfide Stress Cracking. Determination of sulfide stress cracking is covered in NACE TM-01-77 (Ref 98). Stressed specimens are immersed in acidified 5% sodium chloride solution saturated with hydrogen sulfide at ambient pressure and temperature. The solution is acidified with the addition of 0.5% acetic acid, yielding an initial pH of approximately 3. Applied stress at convenient increments of the yield strength is used to obtain cracking data that are plotted as shown in Fig. 51. A 30-day test period is considered sufficient to reveal failure of susceptible material in most cases. Fig. 51 Method of plotting results of sulfide stress cracking tests. Open symbols indicate failure; closed symbols indicate runouts. Source: Ref 98 The purpose of this test standard is to facilitate conformity in testing. Evaluation of data requires individual judgment on several points based on the specific requirements of the end use. Consequently, the test should not be used as a single criterion for evaluating an alloy for use in environments containing hydrogen sulfide or other hydrogen charging elements. Attention should be paid to other factors that may affect SCC, such as pH, temperature, hydrogen sulfide concentration, corrosion potential, and stress level, when determining the suitability of a metal for use. The NACE test method recommends the use of smooth, small-diameter tension specimens stressed with constant-load or sustained-load devices (Ref 98). However, different types of beam and fracture mechanics specimens may be included in the testing standard in the future. Another test method, known as the Shell Bent Beam Test, has been used for over 25 years in the petrochemical industry to rank various materials for use in sour environments (Ref 99). However, acceptance has not been sufficient to generate the interest for standardization. Testing in sodium chloride solution constitutes a worst-case determination for high-strength steels; as such, it is generally considered unrealistically aggressive for the useful ranking of steels in service environments that do not contain hydrogen sulfide or other conditions favoring entry of hydrogen. Tests are usually performed in water containing about 3.5% sodium chloride, artificial seawater, natural seawater (rarely), or a marine atmosphere (Ref 4), unless specific environmental conditions are under study. ASTM G 44 (Ref 8) is used where applicable. In salt water and freshwater, a true threshold K ISCC exists for high-strength steels that is useful for characterizing resistance to SCC. Ideally, K ISCC defines the combination of applied stress and defect size below which SCC will not occur under static loading conditions in a given alloy and environment system. However, the reported value of K ISCC for a given system often reflects the initial K I level and the exposure time associated with the testing. Table 6 illustrates the risk of overestimating K ISCC by terminating the exposure test too soon when using the SCC initiation method (Ref 23, 24). A similar risk exists in tests conducted with the arrest method. Table 7 shows that K ISCC values determined by the initiation and arrest methods may be the same when testing times are sufficiently long and when compatible criteria are used for establishing the threshold (Ref 24). Table 6 Influence of cutoff time on apparent K ISCC using the SCC initiation method Apparent K ISCC Exposure time, h MPa ksi 100 187 170 1000 127 110 10,000 28 25 Note: The initiation method was used on a constant-load cantilever bend specimen (K- increasing) of alloy steel with a yield strength of 1240 MPa (180 ksi). Test environment was synthetic seawater at room temperature. Source: Ref 24 Table 7 Comparison of K ISCC values determined by initiation and arrest methods K ISCC , MPa (ksi ) Steel alloy Initiation Arrest 10Ni, normal purity 24 (22) 26 (24) 10Ni, high purity 59 (54) 57 (52) 18Ni, normal purity 22-33 (20-30) 28 (25) 18Ni, high purity <33 (<30) <33 (<30) Note: Based on a crack growth rate of 2.5 × 10 -4 mm/h (10 -5 in./h). Modified compact specimens: constant load for initiation and wedge-loaded with a bolt for arrest. Test environment: salt water at room temperature. Source: Ref 24 Figure 52 illustrates a method used to compare various high-strength steels (Ref 4, 100). Data were obtained in salt water or seawater, and K ISCC values are plotted versus yield strength. Envelopes are used to enclose all known valid data for the various steels. The crosshatched envelopes or individual data points represent the featured steels, which allows comparison with characteristics of the other steels. The straight lines in Fig. 52 illustrate how K ISCC values relate to the maximum depth of long surface flaws that can be tolerated without stress-corrosion crack growth. Fig. 52 Comparison of SCC behavior of several high-strength steels based on threshold stress intensity (K ISCC ) values in salt water. Source: Ref 100 [...]... 3.1 × 1 0- Bolt-loaded double-beam, pop-in stress; average growth 0-1 5 days 3 .4 × 1 0- Bolt-loaded double beam, pop-in stress; average growth 0 -4 2 days 2.8 × 1 0- m/s in × 105 /h m/s > 54( a) 7 × 1 0-1 0 10(b) 1.3 1 0-1 0 44 7.5 1 0-1 0 11 (c) 48 4 × 1 0-1 0 6 4 × 1 0-1 0 6 40 4 × 1 0-1 0 6 2 × 1 0-1 0 3 9 × in × 105 /h × 2(b) 9 9 Source: Ref 3 (a) Over 4 days (b) Over 6 days (c) No plausible estimate could be made, because... 15 24 221 17 0 3 .4 × 1 0-6 50% sodium hydroxide, 147 °C (297 °F) 51 1503 218 18 . cracks 3 .4 × 10 -6 Air 71 745 108 60 0 Mill annealed 3 .4 × 10 -6 50% sodium hydroxide, 147 °C (297 °F) 61 593 86 52 13 × 10 -5 m (5 mils) 3 .4 × 10 -6 Air 49 15 24 221 . toward sustained-load cracking is greatly reduced (Ref 4, 120). Figure 57 illustrates an example of sustained-load cracking in mill-annealed plate of Ti-8Al-1Mo-1V containing 48 ppm hydrogen this test method (Ref 24, 34) . Fig. 48 SCC propagation rates for various aluminum alloy 7050 products. Double-beam specimens (S- L; see Fig. 28) bolt-loaded to pop-in and wetted three times